ABSTRACT The co-occurrence of mental and physical conditions (?mental-physical comorbidity?) in children is increasing rapidly, disproportionately affecting children covered by Medicaid, and poor quality of care and high costs of care are pervasive issues among this population. Relatively recent proliferation of Accountable Care Organizations (ACOs) is chiefly because these delivery models have offered plausible mechanisms to improve quality and control costs. As such, in 2012 Oregon began enrolling its Medicaid enrollees in Coordinated Care Organizations (CCOs)?Oregon?s version of ACOs. Oregon?s CCOs provide care to more than 90 percent of Oregon Medicaid enrollees; and are regional collaborations between providers, hospitals, and payers; financed under global budgets; charged with controlling Medicaid spending; accountable for integrating medical, behavioral, and dental services; and responsible for measuring and reporting on quality metrics identified by stakeholders. Importantly, CCO incentive and shared-savings payments are tied to their ability to move the dial on cost and quality of care outcomes. Available research indicates CCOs have improved some quality-of-care and cost measures among select populations. Yet, despite their clear potential, no studies have examined the impact of CCOs on quality and costs of care among Medicaid children with mental-physical comorbidity. This research will determine the impact of CCO implementation on quality of care measured by receipt of child and adolescent well-care visits and follow-up after hospitalization for mental illness, and cost of care measured by total payments for hospital services and total payments for ambulatory-care services, among Medicaid children with mental-physical comorbidity. To accomplish this, we will utilize Oregon Medicaid administrative claims and eligibility data for years 2010 through 2015?an evaluation timeframe that will allow us to follow implementation for three years?and a quasi-experimental Difference-in-Differences (DID) analytic approach. Using this approach, the difference in quality and cost of care after CCO implementation among children enrolled in CCOs with mental-physical comorbidity will be compared to that of their counterparts who were not enrolled in CCOs. We hypothesize CCO implementation will be associated with increased quality of care, but exhibit multi- directional effects on cost of care among children with mental-physical comorbidity depending on their underlying service utilization prior to CCO implementation and the type of costs measured (i.e. hospital cost vs. ambulatory care cost). Specific Aim I: Examine the effect of Oregon?s CCO implementation on receipt of child and adolescent well-care visits among Medicaid children ages 3 to 21, with mental-physical comorbidity. Specific Aim II: Examine the effect of Oregon?s CCO implementation on follow-up after hospitalization for mental illness among Medicaid children, ages 6 to 20, with mental-physical comorbidity. Specific Aim III: Identify whether changes in quality of care associated with Oregon?s CCO implementation have had an effect on Medicaid hospital and ambulatory care costs among Medicaid children with mental-physical comorbidity.